1. Field of the Invention
The invention relates to an extruder comprising a casing, at least one bore which is formed in the casing and has a diameter Da, at least one screw which is arranged in the bore in a rotatably drivable manner and has a screw tip arranged at the downstream end—in relation to a direction of flow—said screw tip again being provided with a screw stem having a diameter Di and at least one screw flight, a nozzle plate which is arranged downstream of and adjacent to the screw tip so as to close the at least one bore, and is provided with a plurality of nozzle holes which is formed on an annular surface of the nozzle plate.
2. Background Art
In such an extruder known from EP 1 037 703 B1, a stripper device which is elastically supported in the direction of the axis is attached to the down-stream end of the screw, said stripper device bearing against the inner side of the nozzle plate facing the bore and stripping off material to be conveyed. This embodiment is to be used for conveying polymer pellets containing active ingredients which may be pharmaceutical products, plant protection products, feed additives and food additives.
A screw-type machine which is provided with a stripper bearing against the nozzle plate in an elastic manner is known from U.S. Pat. No. 3,976,799. This screw-type machine serves for the extrusion of food and the like.
A screw-type machine which is provided with a nozzle plate stripped by means of an axially immovable stripper is known from U.S. Pat. No. 2,583,600. This screw-type machine also serves for the treatment of food.
A screw-type machine having a nozzle plate with knives bearing against the inner side thereof, said knives being pressed against the nozzle plate by the material pressure, is known from CH 515 384 A5. The function of these knives, which are pressed against the nozzle plate, is to create predetermined breaking points in the individual strands of material which are created in each nozzle hole, ensuring that the individual strand of material breaks off after leaving the nozzle plate.
These embodiments are not suitable for the extrusion of materials which are capable of flowing only if a substantial amount of shearing action is provided. Such materials are in particular Bingham media where the material does not start to flow until the flow limit, i.e. a particular shear stress, is exceeded. Such materials are for example plastic ceramic compounds, colloidal dispersions, slurries of glass beads, etc.